Effect of exogenous progesterone and eCG treatment on ovarian follicular dynamics in vicunas ( Vicugna vicugna)

Abstract

The aim of the present study was two-fold. First, to evaluate the effect of exogenous progesterone on ovarian follicular dynamics in order to assess its ability to synchronize ovarian activity in the vicuna. Secondly, to evaluate the ovarian response to the treatment with eCG through the observation of the structures developed in the ovaries. Follicular dynamics was monitored daily by transrectal ultrasonography in 12 adult, non-pregnant vicunas. Plasma progesterone and estradiol-17β concentrations were measured in blood samples collected daily. In experiment 1, intravaginal devices containing 0.33 g of progesterone were inserted into the vagina and kept in place for 5 days (treatment group, n = 8). After progesterone withdrawal, five animals were further monitored in order to evaluate the efficacy of the CIDR ® to synchronize the emergence of a dominant follicle. In experiment 2, four females received 750 IU of eCG IM. Two were previously monitored ultrasonographically to confirm the absence of a dominant follicle at the beginning of the superstimulatory treatment (group A). The other two animals had a CIDR ® inserted into the vagina for 5 days and the superstimulatory treatment was applied 24 h after device withdrawal (group B). Females from both groups were surgically explored 96 h after eCG injection; the ovaries were exposed and the number of newly formed structures produced by each ovary was counted. Peak progesterone concentrations (25.9 ± 5.29 nmol l −1, mean ± S.E.M.) were attained on day 1 after device insertion, remained high until the day of device withdrawal (9.7 ± 1.98 nmol l −1) and decreased to 5.5 ± 1.13 nmol l −1 the day after. There was no follicle development to the state of dominance after device insertion. Moreover, mean follicle diameter steadily decreased after insertion of the device until the minimum mean value (1.85 ± 0.17 mm) was recorded on day 5 ( P = 0.006). Similarly, plasma concentrations of estradiol-17β remained below 35 pmol l −1 during the period of progesterone treatment in all animals and the mean estradiol-17β declined with the lowest value (22.1 ± 2.19 pmol l −1) being recorded on day 4 after device insertion. After superstimulation of follicular development with eCG, the total number of follicles that developed was 33 in group A and 58 in group B and the mean number of newly developed ovarian structures per female was 22.75 ± 4.26. In conclusion, progesterone released by the CIDR ® exerts a negative effect on ovarian follicular development and function suggesting intravaginal devices could be used to synchronize the beginning of follicular waves during a superstimulatory treatment. There was also a tendency for greater ovarian follicular development when the animals were previously treated with progesterone.